Net Loss Per Common Share (Tables)

12 Months Ended

Dec. 31, 2012

Net Loss Per Common Share

Schedule of computation of basic and diluted net loss per share available to common stockholders

	Years ended December 31,
(in thousands, except per share amount)	2010			2011			2012
Net Loss Per Share:
Numerator:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(50,104	)	$	(79,223	)	$	(91,277	)
Plus: Unaccreted dividends on convertible preferred stock		(11,185	)		(7,789	)		(2,107	)
Net loss available to common stockholders—basic and diluted		(61,289	)		(87,012	)		(93,384	)
Denominator:
Weighted-average common shares—basic and diluted		10,994			11,343			72,831
Net loss per share available to common stockholders—basic and diluted	$	(5.57	)	$	(7.67	)	$	(1.28	)

Schedule of potentially dilutive securities excluded from computation of diluted weighted average shares

	Years ended December 31,
(in thousands)	2010		2011		2012
Convertible preferred stock		55,253		66,256		—
Options to purchase common stock		16,214		17,617		18,066
Convertible preferred stock warrants		306		302		—
Common stock warrants		2,937		2,640		2,842